Cofiring and Plant Conversion Flashcards
What incentives are there for cofiring
Reduction in emissions of fossil fuel generated CO2
Only modest capital investment required
Utilising existing infracstructure
Contracts for Difference
What is the potential in co-firing
UK lead in development, incentivising full conversion to biomass and coal phase out by 2025.
Takes advantage of efficiency and economics of large scale boilers
Supports economic development among wood and energy crop producers, but sustainable wood sources must be used
What are the risks in co-firing plants?
Supply chain is more complex and possibly less secure than fossil fuels
Reduction in plant availability and flexibility
Increased maintenance and replacement costs for handling equipment.
What are the effects on the equipment for co-firing?
Grinding: reduces capacity and lifetime
Combustion: Slagging
Superheater: Corrosion
HX: fouling and erosion
Treatment: Capacity and catalyst poison
Pre-mixing of biomass and coal
Easiest approach requiring minimal capital.
Requires additional infrastructure for reception, storage and conveying.
Limited by volume of existing conveyors (biomass has lower density)
Separate conveying to the mill
Mill has to be modified to operate with cold primary air.
Mill breaks pellets to original size.
Biomass can form a mat
Poor mill performance, risk of fire and limited to 3% thermal.
Torrefaction can increase density and storage potential
Why would separate biomass hammer mills be installed
Hammer mills are not robust, best co2 reduction to cost ratio.
Minimal outage, more flexible and higher throughput.
Explain the process of Drax power station
1.5 tonnes of pellets by direct injection
Pellets discharged over a grate into underground storage and conveyed to silo
Conveyed to a final screen and pneumatically delivered to coal pipe
Max 12.5% heat by co-firing
Pellets favoured to maximise the volume
What occurs in the studstrup power plant
Straws are de-baled then conveyed through hammer mill. Pneumatically conveyed to the modified coal burners.
Oil gun removed from centre and straw is blown through open pipe at centre. Then 50% of coal is blown around the outside of the tube
What are the 2 types of indirect cofiring
Parallel: Combustion in separate combustor and boiler
Gasification: Gasification and combustion of product fuel gas
Give an example of parallel combustion site and how it works
Avedore
Biomass reception: straw lines and feeding system handling bales.
Transported from local farms on lorries, unloading area moisture detector.
Auto-cutting of strings then deesing into peg rollers loosen material before it is blown.
Solid residue is kept separate and used as a fertiliser.
Coal ash from unit 1 used in cement industry.
A solo straw boiler would have low efficiency (35%) but increases to 46% by sharing the large steam turbine
Consideration for transport of biomass via ship
Wood pellet must be kept dry so unloading must be protected from rain or sea spray
Transferred by crane grab/pneumatic unloader
Large amounts of pellets introduce fire risk (bulk heating), explosion (dust) or CO poisoning (off-gas)
Considerations for rail transport
Similar to coal wagons but weather proof doors fitted
Bottom discharge doors allow the pellets to be dropped automatically when fixed at low speed
Road considerations
Cost significantly more for bulk materials in continuous flow.
Lartger carbon footprint, reducing the rationale of using biomass
Rail loading is faster, automated and more efficient
Storage risks and design
Explosion (high dust conc at top of silo)
Fire (self heating)
First in first out to minimise dwell time and reduce degradation
Storage controls
Explosion vents which open with explosive pressure to protect structural integrity
Monitoring of CO and temperature.
Foam blanket suppression for fire
What are some ignition sources for fire
Stones and metal producing sparks
Equipment (sparks, overheating, faults)
Mechanical bearings (reduce use of friction bearings)
Long term static pockets of material (self heating)H
How to mitigate fire hazards
removal of contaminants
use safe electrical systems (ATEX)
Temperature and off-gas monitoring
Barriers to prevent explosion
Inert gas/powder suppression and foam extinguishing system
What are some problems with fuel switching? 7 problems
High moisture - lowers flame temperature and reduces efficiency.
Lower CV - more fuel needed
Lower density - more storage and transport space
Poor milling - separate mills required
Degrades in storage - silos and underground needed
Hygroscopic - absorbs moisture, leading to self heating
Different chemicals in the ash - slagging, fouling, corrosion
Why can fluidised beds allow biomass with higher moisture?
thermal mass of the hot bed material
How does ash create problems
High alkali content results in ash with different melting characteristics
Alkali vapours combine with sulphur and silica, which form low melting compounds which bind ash particles to grates and Hx surfaces
What is slagging, fouling and corrosion?
Slagging - melting of ash in furnace
Fouling - accumulation of sticky ash particles
Corrosion - gradual destruction of the boiler
